This invention relates to a thin film EL element, particularly a thin film EL element with an improved insulating layer, and to a process for producing the same.
Basic cross-sectional structure of a conventional thin film EL element is shown in FIG. 7, where a transparent electrode 2 composed of In.sub.2 O.sub.3, SnO.sub.2, etc. is laid on a glass substrate; a first insulating layer 3 of Y.sub.2 O.sub.3, Si.sub.3 N.sub.4, SiO.sub.2, etc. is formed thereon by sputtering or electron beam vapor deposition; and a light-emitting layer is further formed thereon. In case of yellowish orange light emission, the light-emitting layer can be formed by electron beam vapor deposition or sputtering of sintered pellets of ZnS doped with Mn as a dopant. As other sintered pellets for the light-emitting layer, sintered pellets of ZnS doped with TbF.sub.3 or TbP as a dopant for green light emission, those doped with SmF.sub.3 for red light emission and those doped with TmF.sub.3 for blue light emission are available. A second insulating layer 5 composed of the similar material to that used in the first insulating layer 3 is laid on the light-emitting layer 4, and further a back side electrode 6 composed of Al, Au, etc. is formed thereon by vapor deposition. An AC power source is connected to the transparent electrode 2 and the back side electrode 6 to drive the thin film EL element. Some of thin film EL elements of such a double insulating layer structure are now commercially available, but still have such problems as a low emission brightness, a high driving voltage, a low emission efficiency and a low reliability of element performance. Thus, research and development of thin film EL elements have been now extensively made for improvement of light emission characteristics, that is, a higher emission brightness, a lower driving voltage, a higher emission efficiency and a higher reliability of element performance.
Such thin film EL elements are disclosed, for example, in Japanese Patent Application Kokai (Laid-open) Nos: 58-216391 and 52-129296; Shingaku Gizyutsu, Vol. 85, No. 32, ED85-6 (May issue, 1985).
Improvement of the light emission characteristics of the thin film EL elements depends upon how to prepare a high quality insulating layer or how to select a most appropriate insulating layer structure, because the light emission of the thin film EL element takes place when an electric field of about 8.times.10.sup.5 to about 1.times.10.sup.6 V/cm is applied to the light-emitting layer, and thus the insulating layers must have a dielectric breakdown strength (dielectric strength) above that value.
As the material for the insulating layer, SiO, SiO.sub.2, Y.sub.2 O.sub.3, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, etc. have been so far used, but the portion of applied voltage shared by the insulating layer is very large owing to the low dielectric constant of these insulating layer materials. Thus, a very high driving voltage is required for emitting the light-emitting layer.
In order to lower the driving voltage, thin film EL elements using insulating layer materials having a higher dielectric constant than those of SiO.sub.2, Ta.sub.2 O.sub.5, etc., that is, SrTiO.sub.3, PbTiO.sub.3 and BaTa.sub.2 O.sub.6 have been investigated. However, SrTiO.sub.3, PbTiO.sub.3 and BaTa.sub.2 O.sub.6 have a higher dielectric constant, but are liable to undergo dielectric breakdown and thus have a low dielectric breakdown strength.
Generally, the dielectric breakdown strength of these insulating layer materials greatly depends upon conditions for forming the insulating layer, and thus how to prepare a high quality insulating layer is an important task.
The conventional thin film EL element using an insulating layer material having a low dielectric constant requires a very high driving voltage, whereas a thin film EL element using an insulating layer material having a high dielectric constant has a low dielectric breakdown strength.